1. Field of the Invention
The present invention relates to filter cartridges generally and, more particularly, to filter cartridges having a plurality of filter cells which cells include internal cell separators.
2. Description of the Prior Art
Cell type filter cartridges typically are fabricated from individual cells which generally have two layers of filter medium separated from each other so that liquid flows from the outside of the filter medium into the space between the layers of filter medium towards the central portion of the cell. Often the separators that keep the layers of filter medium apart are formed in the shape of discs with ribs radially extending from the central aperture in a spoke-like pattern. The ribs are widest near the central aperture of the cell.
The typical construction of the separator discs poses two problems. First is the loss of filtering area. Though the discs do separate the upper and lower medium layers of the cell, they do not provide rigidity or stiffness to the cells. Therefore, in cartridges having a plurality of cells, the cells tend to droop onto each other so that the adjoining media surfaces will contact each other thereby reducing the effective filtering area of the cartridge. In order to compensate for the lack of stiffness or rigidity in the filter elements, external spacers have been put into the cartridge type filters to separate the cells and maintain them in their proper position for maximum filter area.
The second problem is the loss of fluid flow area through the cell. The ribs of the separator discs within each cell have the narrowest spacing at the ends which form the central aperture of the disc, i.e., the minimum flow area occurs at the point of maximum flow. Therefore, the velocity of the fluid must be greatest at this point of minimum flow area and this high velocity of the fluid flow tends to abrade the filter media. The rib ends extend above any connecting member and contact the inner surface of the media. The media is forced down upon the rib ends by several different forces. First, the swelling of the media during use. Second, the axial forces exerted on the cartridge to force the cells together or sealing means therebetween to prevent seepage of unfiltered liquid between the cells. Third, the hydraulic pressure of the liquid to be filtered forces the filter media against the disc.
The forcing of the media down on the ends of the separating ribs has serious consequences. It forces the filter media down between the separating ribs at the central aperture. This further restricts the passages formed by the separating ribs between the layers of filter medium on each disc. This produces increased resistance to flow as well as increased media abrasion and migration resulting from the high fluid flow velocity in contact with the distorted and damaged filter media at that point.
The attempts to maintain the separation of the cells and other examples of attempts to avoid the problem of media intrusion are shown in several patents, though none have satisfactorily addressed these concerns. For example, U.S. Pat. No. 3,666,107 discloses a filter cartridge comprising a plurality of filter elements each consisting of two dished diaphragm thereto one or more circumferential or radial stiffening members or ribs. Such construction is believed to be difficult to manufacture and would impair to some extent the usable surface area of the filter elements.
A prior art filter cartridge having wafer-like elements but with no additional support therefor is seen in U.S. Pat. No. 2,966,269. This patent discloses a filter assembly including filter cartridges, each constituting a stack of wafers, each wafer being formed from a pair of opposed dished annular discs of filter paper or the like. The discs are secured around their outer peripheral edges, for example by an adhesive, and are spaced apart at their inner peripheral edges by annular spacers between the discs, i.e. within the wafers. The annular spacers have apertures opening into the interior of the wafer. The wafers are stacked together in generally cylindrical form and held together between end plates by tie rods.
Other filter cartridge structures are disclosed in U.S. Pat. Nos. 3,165,471; 4,094,793; 1,102,685; 1,742,919; 2,411,341; 3,178,028; and 4,048,073.
Another example of an improved separator for a cell type filter is shown in the co-pending application, Ser. No. 06/803,819, filed Dec. 2, 1985, which was issued as U.S. Pat. No. 4,704,207, for a filter cartridge including external cell separators, assigned to the assignee of the present invention.